JPS61217668A - Precipitated silica heat-insulating material - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION Background of the Invention The design and development of insulation materials, such as insulation materials for refrigeration structures,
It is a widespread technology. A number of devices have been developed using various fibrous and powdered materials as insulation materials.

There are a wide variety of ways to utilize such insulating materials, including the use of exhaust shells and bags, compression of the insulating material, and various orientations of the material. Many of the insulating materials developed to date have been found to be well suited for their intended purposes, examples include U.S. Pat. No. 2,768,046, U.S. Pat.
No. 9549.

The prior art as described above discloses the use of a powdered heat insulating material sealed in a container of an appropriate type. For example, in an insulated panel for refrigerators and freezers described in US Pat. No. 2,989,156, the panel is formed by sealing two metal sheets, evacuating the center, and filling with expanded perlite. US Patent No. 43
No. 99175 describes the use of pressure molding of a powdered insulating material placed in an outer container to form an insulating board.

A thermal insulation device is described in U.S. Pat. No. 4,349,051 in which a powdered thermal insulation material is sandwiched in the space between a rigid outer wall and a flexible inner wall. A similar structure is disclosed in U.S. Patent No. 316651.
It is disclosed in No. 1.

Furthermore, those in which a metal shell filled with a powder material is hermetically sealed are disclosed in U.S. Pat.
No. 64143.

The use of artificially produced silica materials as insulation materials is described in US Pat. No. 4,159,359.

The silica material described in this patent is fumed silica, which is formed by heat treating a silane material to produce the desired silica particles. This fumed silica is a relatively expensive synthetic silica, and its specific parameters are described in the above patent specification.

What is noteworthy about U.S. Pat. No. 4,159,359 is that the patent specification mentions that precipitated silica
ted 5ilica) powder has too high thermal conductivity,
Therefore, it is stated that it poses no problem as a heat insulating material.

In accordance with the present invention, it has been surprisingly found, especially in light of the disclosure of the above-mentioned US Pat. No. 4,159,359, that precipitated silica can be used to form excellent thermal insulation materials.

In addition to the thermal insulation properties of precipitated silicas which are often superior to those of fumed silica, precipitated silicas are significantly lower in cost and therefore provide superior performance at lower cost.

Precipitated silica according to the invention is produced by means well known in the art by the interaction of alkaline water glass with mineral acids. The precipitated silica thus obtained is then mechanically processed, such as by spray drying and milling, to obtain the desired particle size and surface area.

The precipitated silica thus prepared is heated to drive off surface water. In some cases, it has been found advantageous to place the silica in a microporous pouch which constitutes a simple container during this heat treatment.

After the precipitated silica powder has dried, it is placed into a plastic jacket, preferably a plastic jacket provided with a metal coating or a metal foil layer, and then the jacket is evacuated and sealed. If the precipitated silica is dried in a microporous bag, the microporous bag can be placed directly into a plastic envelope. Before or during evacuation of the plastic envelope, the precipitated silica is compressed to the desired density, thereby
Excellent thermal insulation with a sufficiently thin structure can be obtained at low cost. After compression and evacuation, the plastic jacket containing the compressed precipitated silica is essentially in a board-like shape, and such jackets can therefore be easily attached to structures that require insulation, such as the walls and doors of refrigerators and freezers. can be placed in

By using the materials and processing methods of the present invention, insulation 4I can be obtained that is thinner than conventional refrigerator and freezer insulation for the same insulation value, and with better insulation efficiency for the same thickness.

DETAILED DESCRIPTION OF THE INVENTION As previously mentioned, the insulating material of the present invention is made from precipitated silica. To produce the precipitated silica, an alkaline silicate is first treated with a mineral acid. The product thus obtained is spray dried, milled and then heated to drive off the surface water. The thus dried silica is then placed within an essentially airtight and watertight envelope where it is vacuumed and pressurized to form a material having a board-like stiff consistency. This material is typically 0.5 to 1 inch thick and flat. The length and width of the panel obtained by the above process is limited only by the dimensions of the components of the particular device, such as a freezer or refrigerator, into which the panel is inserted.

When the insulating material of the present invention is manufactured in duplicate, the resulting panels have a value of approximately 0.05 (BTU) (IN).
/ (HR) (FT) 2 (cho) or less was confirmed. It has previously been established that values in this range are desirable for refrigerator and freezer manufacturing. The insulation material produced according to the invention therefore allows for thinner walls for the same heat leakage and thus for smaller external dimensions or larger internal volumes of the parts of the equipment to be insulated, or for the same wall thickness. can form more energy efficient components of the device.

As mentioned above, the precipitated silica of the present invention is produced by the interaction of alkaline water glass and mineral acid. Preferably, the alkaline water glass is sodium water glass and the mineral acid is sulfuric acid. When the white precipitate resulting from the interaction between the two is washed and dried, the silicon dioxide content is usually 86%.
A product is obtained in which the remainder is mostly water, with a small amount of salt residues formed during the reaction and traces of metal oxides. A variety of precipitated silicas are available commercially, with varying properties depending on the composition and ratio of reaction materials, reaction time, temperature and concentration. The subsequent treatment of the white precipitate also influences its properties; such post-treatments include passing, drying in various ways, grinding or milling in various ways, and classification. Properties influenced by the type of treatment, both in precipitate interaction and in post-treatment, include surface area, particle size and density. In general, a surface area of 150 m2 (square meter)/2 (Durham) or more has been found to be useful for the present invention, as determined by the BET method (DIN 66 131). Furthermore, the silica used must normally be neutral but slightly alkaline (pH 6.0 or higher). The particle size of the precipitated silica agglomerates (aggloa+erate) used according to the invention is 50
It is preferably less than μll1 (micrometer), particularly less than 10 μm.

To form an insulating panel according to the present invention, the hand precipitated silica is first dried by commercial routes.

If desired, the silica may be placed in a microporous bag. This bag is only used as an aid to hold the silica powder during the drying process. If it is desirable to use such a microporous material, possible materials include polypropylene sold under the trade name Celgard by Celanese Ff. You can use different types of paper.

In general, any material that allows air and moisture to pass through, but is capable of retaining finely divided silica, will suffice.

During the drying process, whether or not microporous bags are used, the temperature must be sufficient to drive off surface water. Generally this is about 10 when using microporous bags.
It refers to the temperature of 0°C, and its -t limit is the temperature at which the microporous material carbonizes.
This is the temperature at which it will not melt or decompose.

After the drying process, the dried silica is pressurized to about 10 to 2
Cake having a density in the range of 0 lbs/ft3, preferably 10 to 13 lbs/ft3.
) to form. The material used according to the invention has a density of 0.05 (BTU) (IN)/(HR) (
FT) 2 ("F) or less. Place the dried silica in a separate plastic jacket or bag. This plastic jacket should be shaped to prevent gas leaks. If it dries in a porous bag, place the microporous bag directly inside the plastic jacket.Usually, a thin metal foil layer is used to prevent gas leakage in the plastic jacket. For example, the plastic envelope used in the present invention may be formed from six layers of a polymer such as polypropylene, three of which are Plastic envelopes of the type in which the layers are coated with aluminum to form a gas barrier have been found to be useful.

The overall thickness of the plastic jacket is small enough to
There should be little heat conduction through the side edges. Generally, the overall thickness should be about 0.003 to 0.020 inches. Although thinner materials may still be strong enough to retain the silica and withstand the necessary subsequent processing, the expected lifetime of equipment in which this material is placed may be shortened. However, even with a jacket thickness of 0.003 inches, an expected life of more than 5 years can be expected.

After the dried silica is placed in the plastic envelope, the envelope is evacuated and sealed by suitable means, such as heat sealing or adhesive. It is desirable that the internal pressure be 10I10lll1 or less, but depending on the filling material, a slightly higher pressure, for example about 15fflff1, may be sufficient. The necessary degree of vacuum is determined based on the value, which must be 0.005 or less, as described above. If desired, an inert gas such as carbon dioxide or nitrogen may be used to drive air from the envelope prior to evacuation.

Examples of the present invention are shown below. These examples are illustrative only and should not be considered as limiting the scope of the invention in any way.

Example I To make an insulating panel, approximately 300 pieces of precipitated silica were first placed in a microporous bag. The microporous bag is made of a material sold under the trade name Celgard, and the precipitated silica is manufactured by Degussa.
A product sold under the trade name rFK-310J from a) was used. This precipitated silica has a surface area of 650 m2/'
(by BET method), average aggregate particle size 5μ11, tapping density 130゛/I! , pH
7, DBP absorption 210 and sieve residue 0.01 (
according to DIN 53 580). After the precipitated silica was placed into the microporous bag, the open sides of the microporous bag were heat sealed and then placed in an oven and held at 96°C for 16 hours.

The dried silica in the microporous bag was then placed in a metal coated plastic jacket with vent holes. The jacket used had six layers of laminated polymer membranes, three of which were coated with aluminum, as described above, and the total jacket thickness was 0.004 inches. After placing the microporous bag within a metallized plastic jacket, the jacket was sealed except for the exhaust vents and the panel was heated to a density of 19.4 while evacuated to 0.7 Torr. Thickness 0 in pounds per cubic foot
．． Compressed to 626 inches.

When the obtained panel was placed in a thermal conductivity tester and measured, the C value was 0. 066 (BTU) / (HR) (
FT)2 (bottom), and the K value is 0.041 (BTU)
(IN)/(HR)(FT)2 (bottom).

Example ■ The influence of vacuum on insulation properties was investigated. Another precipitated silica sold by Degussa under the tradename rFK500-LSJ was prepared as in Example I. The density of the final drizzled product was 12 pounds per cubic foot and the final panel thickness was 0.715 inches. When a vacuum leak was slowly caused, the data shown in Table 1 was obtained. In Table 1, the unit of C is (BTU)/
(HR) (FT)2 (di), and the unit of K is (B
TU)(IN)/(HR)(FT)2 (bottom).

Table 1 Examples ■ Other precipitated silicas were tested using otherwise the same conditions and materials as used in Example I. The results obtained are shown in Table ①. In Table ■, the unit of packing density is (
LB)/(FT)3, unit of internal pressure is 111H'),
The unit of K is (BTU) (IN)/(HR) (FT)2
(bottom).

In Table 2, Sibernut (S1pcrnat) is a product name of Degussa.

Hl-SIL and Lo-vc
l) is the trade name of PPG ratio.

In addition, when the same test as FK310 was conducted, the above Gibelnut 22S had a BET surface area of 1901, an average lump particle size of 7, a tapping density of 120%/1, and a pH of 6.3.
, DBP absorption 270, sieve residue 0.1. Gibelnut 22LS has a BET surface area of 170, an average agglomerate particle size of 465, and a tapping density of 80. pH 5.3, DBP absorption 2701 sieve residue 0.01. Gibernut 5
0 means BET surface area: 4501, average lump grain size: 50, tapping density: 200. pH 7, DBP absorption 340, sieve residue 0.5. Gibelnut 50S has a BET surface area of 450, an average agglomerate particle size of 8, and a tapping density of 100.
pH 7, DBP absorption 330, sieve residue 0.1.

Hisil T600 has a median bulk particle size of 1.4 μl, an average final particle size of 21 nm (nanometers), and a surface area of 150 m.
2/'), pH 7.0, and bulk density (by tapping) of 3 to 4 pounds per cubic foot.

The use of precipitated silica as an insulating material for various devices, particularly cooling devices, has been described in accordance with the present invention. It also showed many types of precipitated silica. The invention should not be considered limited to the particular embodiments, but rather as defined in the claims.

Claims (1)

[Scope of Claims] 1. (a) Fine silica material obtained by precipitating silica by interaction of alkaline water glass and mineral acid and then drying to form dry fine silica; (b) An airtight and watertight outer cover containing this dry fine silica, and a board-like material plate for insulation. 2. The board-like material sheet according to claim 1, wherein the alkaline water glass is sodium water glass and the mineral acid is sulfuric acid. 3. The board-like material plate according to claim 1, wherein the fine silica is placed in a microporous bag and dried. 4. The board-like material plate according to claim 3, wherein the microporous bag is arranged within the airtight and watertight jacket. 5. The board-like material of claim 1, wherein said airtight, watertight jacket containing said dry, finely divided silica is compressed to a density of 10 to 20 pounds per cubic foot. 6. The board-like material of claim 5, wherein said density is between 10 and 13 pounds per cubic foot. 7. (a) Precipitating fine silica by the interaction of alkaline water glass and mineral acid, (b) drying this fine precipitated silica at a temperature sufficient to drive off surface water, and (c) this drying. 10 to 20 pounds of precipitated silica
compressed to a density of cubic feet; (d) placing the dried silica in an air-tight, water-tight jacket with vents; (e) evacuating said air-tight, water-tight jacket; and (f) evacuating said air-tight, water-tight jacket. A method of forming a thermal insulation material including steps of sealing an exhaust hole in a jacket. 8. The method for forming a heat insulating material according to claim 7, wherein the alkaline water glass is sodium water glass and the mineral acid is sulfuric acid. 9. The method for forming a heat insulating material according to claim 7, wherein the precipitated silica is placed in a microporous bag and dried. 10. The method of forming a heat insulating material according to claim 7, wherein the airtight and watertight outer jacket is made of a multilayered plastic structure including a metal coating layer. 11. The method of forming a heat insulating material according to claim 7, wherein the drying temperature is about 100°C. 12. The method of forming a heat insulating material according to claim 10, wherein at least one of the metal coating layers is an aluminum-coated plastic layer. 13. The method of forming a heat insulating material according to claim 10, wherein at least one of the plastic layers is made of polypropylene. 14. The method for forming a heat insulating material according to claim 10, wherein at least one of the layers is an aluminum foil layer. 15. The method according to claim 7, in which the internal pressure is reduced to 15 torr or less by evacuation. 16. Insulating material composed of fine silica produced by sedimentation. 17. The heat insulating material according to claim 16, wherein the precipitated silica is formed by interaction between alkaline water glass and mineral acid. 18. The heat insulating material according to claim 17, wherein the water glass is sodium water glass and the mineral acid is sulfuric acid. 19. The heat insulating material according to claim 16, wherein the silica has a lump particle size of 50 μm or less. 20. The insulation of claim 16, wherein said finely divided silica is compressed to a density of 10 to 20 pounds per cubic foot. 21. The insulation material of claim 20, wherein said density is between 10 and 13 pounds per cubic foot.